Metal oxide-containing green pellet for reducing furnace, method for production thereof, method of reduction thereof, and reduction facilities

ABSTRACT

The present invention provides a method and facility for preventing crumbling and powderization of green pellets when producing high strength green pellets using a powder feedstock and using the pellets in a rotary hearth reducing furnace and for efficiently reducing the same. It comprises kneading by a kneader  5  a feedstock of a powder of a fine particle size (20 to 80 wt % having size of not more than 10 μm) including a metal oxide and carbon-bearing powder fed from a feed storage tank  1  and producing green pellets by a pan type pelletized  7.  The green pellets are screened by a pellet screen  9,  then dried by a pellet dryer  11  and reduced by firing in a rotary hearth reducing furnace  13.  At that time, the green pellets are continuously conveyed to prevent crumbling.

TECHNICAL FIELD

[0001] The present invention relates to metal-oxide bearing greenpellets for a reducing furnace produced as an intermediate when reducingmetal oxides of powderized ores etc. and dust or sludge containing metaloxides generated in metal refining industries and metal processingindustries, a method of production of the same, a method of reduction ofthese green pellets by a rotary hearth reducing furnace or otherreducing furnace, and a reduction facility. Note that in the presentinvention, “green pellets” are pellets before reduction in a reducingfurnace, while “reduced pellets” are pellets reduced in a reducingfurnace.

BACKGROUND ART

[0002] There are various types of metal reducing processes for producingreduced iron or alloy iron. Among these, there is a process using powderof a metal oxide as a feedstock, producing spherical pellets, andreducing these at a high temperature. As examples of this type ofprocess, there are shaft type hydrogen gas reducing furnaces, rotarykiln reducing furnaces, rotary hearth reducing furnaces, etc. The greenpellets used in shaft type hydrogen gas reducing furnaces are obtainedby forming powder ore into grains and reduced by hydrogen gas. On theother hand, in rotary kiln reducing furnaces or rotary hearth reducingfurnaces, heat is supplied from the reducing furnace and the reductionreaction is performed by the carbon mixed in the green pellets. That is,in a rotary kiln reducing furnace or rotary hearth reducing furnace,pellets comprised of the carbon of coal, coke, etc. mixed with the metaloxide powders are used. These processes can use inexpensive coal etc.,so have caught attention as economical methods of production of reducediron. Further, they are being used as high productivity processes inrotary hearth reducing furnaces.

[0003] A rotary kiln is a furnace comprised of a rotating cylinderhaving a diameter of 2 to 5 m and length of 30 to 80 m. The cylinder ismade of steel and is lined with refractories. The furnace temperature is300 to 600° C. at the feed part and about 1100° C. at the exit. Thegreen pellets fed are heated over about 6 hours to about 1100° C. Atthat temperature, the carbon and metal oxide in the green pellets reactto produce carbon monoxide and metal and form the reduced pellets. Thereduced pellets are discharged from the kiln and cooled. After this,they are used as the feedstock for electric furnaces or blast furnaces.

[0004] A rotary hearth reducing furnace is a reducing furnace of a typehaving a disk-shaped hearth of refractories with a cutaway centerrotating on rails at a constant speed under a fixed ceiling and sidewalls of refractories (hereinafter referred to as a “rotary furnace”).The diameter of the hearth of the rotary furnace is 10 to 50 meters andthe width of the hearth 2 to 6 meters. The green pellets are fed to beuniformly spread on the hearth of the rotary furnace. The hearth rotatesand moves the parts of the furnace, that is, the feed part, the heatingzone, the reducing zone, and the discharging part, along with the greenpellets. The green pellets are charged into the about 1000° C. hightemperature feed part. Next, they are heated at the heating zone byradiation from the high temperature gas to about 1200° C. or more, thenthe carbon and metal oxide in the green pellets react at the reducingzone, whereby reduced metal is produced. In a rotary hearth reducingfurnace, since the heating is quick, the reaction ends in 7 to 20minutes. The reduced pellets are ejected from the furnace and cooled,then used as feedstock for electric furnaces or blast furnaces.

[0005] Further, in a rotary furnace, since the green pellets are placedstationarily on the hearth, there is the advantage that the pellets areresistant to crumbling in the furnace. As a result, there are the strongpoints that there are little problems of powderized feed sticking to therefractories and the yield of the nuggets is high. Further, there arethe advantages that the productivity is high and an inexpensive orcoal-based reducing agent or powder feed can be used.

[0006] As explained above, in a rotary furnace, the green pelletscomprised of a powder feed including metal oxide and carbon shaped intograins is spread on the rotary hearth where it is reduced by heating.The green pellets are placed relatively stationarily on the hearth. As aresult, easy to handle pellet-shaped reduced metal is obtained. Ifpowder agglomerates like pellets, the contact between the metal oxideand carbon is good and the reduction reaction easily proceeds actively.

[0007] In this way, in these processes, powder comprised mainly ofcarbon and metal oxide is shaped into green pellets and these greenpellets are used as feedstock for reduction by heating

[0008] As the feedstock for the green pellets, powder ore, metal oxidedust, or other metal oxide and carbon as a reducing agent are used. Inthe production of reduced iron, pellet feed or other fine iron ore isused. The reducing agent used is carbon, but the ratio of the carbon notvolatilizing (fix carbon) until the temperature where the reductionreaction occurs, that is, about 1100° C., is preferably a high one. As asource of such carbon, coke fines or anthracite is good.

[0009] In general, powders of two or more types of feedstock are used.This is for adjusting the ratio between the metal oxide and carbon. Forproduction of green pellets, a pan type pelletizer is used. First, thepowders of the feedstock are mixed in predetermined ratios, then theresult is shaped into green pellets by the pan type pelletizer.

[0010] A pan type pelletizer is comprised of a rotary pan of a diskshape having a diameter of 2 to 6 m. The pan is inclined at about 45degrees. A feed powder including water tumbles inside it. While thishappens, the feed powder is coated around the generated nuclei whichthen grow into green pellets. The sufficiently grown green pellets leavethe pan by their own weight.

[0011] When the rotary furnace is a rotary kiln, the green pellets arefed into the furnace without drying. This is because the temperature ofthe feed part of the rotary kiln is about 300° C. and the green pelletswill not burst in the moist state. On the other hand, when a rotaryhearth reducing furnace, the temperature of the green pellet feed partis 1000° C. or more, so green pellets containing moisture will burst dueto evaporation of the moisture and therefore the green pellets are driedbefore being fed to the furnace.

[0012] For the feed powder containing the metal oxide (hereinafterreferred to as the “metal oxide-bearing powder”), generally an ore isused, but sometimes use is made of iron-making dust or thickener sludgegenerated in the process of refining in a blast furnace, converter,electric furnace, etc. or the process of rolling and processing. Inparticular, dust or sludge generated in the steelmaking industryincludes impurities such as zinc or lead, but these evaporate along witha reduction reaction of 1200° C. or more, so this reaction is aneffective means for removal of impurities. It is also used as a processfor treating dust containing large amounts of impurities and iseffective for recycling of metal resources.

[0013] In this way, in the process for reducing green pellets, forstable operation, a high strength of the green pellets used as thefeedstock is important. For example, with a vertical shaft furnace, ifthe green pellets are insufficient in strength, powder generated bycrumbling of the green pellets will enter between the green pelletsstacked in the furnace leading to the problem of obstruction of the flowof gas, the problem of too much dust trapped in the dust trap, etc. Inthe case of a rotary kiln, if the green pellets are insufficient instrength, the green pellets will crumble when tumbling in thekiln—leading to the problem of the dust generated at that time stickingto the refractories and creating a dam ring. As a result, the greenpellets will not pass over the dam ring and green pellets will no longerflow through the inside of the kiln.

[0014] Further, in the case of a rotary hearth reducing furnace, if thegreen pellets are insufficient in strength, the green pellets willcrumble, and the metal oxide powder of the crumbled green pellets willaccumulate on the hearth and be heated and sinter at a high temperatureof 1200° C. or more. The sintered powder will bond together and willsinter and bond with the refractories of the hearth as well to stick tothe hearth. The stuck powder will build up on the hearth and cause wearon the blade of the screw-type discharger discharging the reducedpellets on the hearth. As a result, the blade lifetime will becomeextremely short. A blade which has a lifetime of at least one year inusual operation will sometimes have to be replaced in one month.Further, along with buildup on the hearth, it will become impossible tospread the green pellets on the hearth normally. To eliminate thisproblem, it is necessary to cool the entire furnace and use a breaker orother machine to break up and remove the buildup on the hearth. As aresult, each time, the facility will have to be idled for at least fivedays. This is a problem causing a major drop in the operating rate.

[0015] In this way, if the green pellets are insufficient in strength,the operation of the reduction process becomes unstable. Therefore, tosolve these problems, green pellets generating little powder have to befed. Technology for producing high strength pellets under stableconditions has therefore been sought. In particular, this demand hasbeen acute for pellets using powder containing carbon (powder coal,coke, charcoal, etc., hereinafter referred to as “carbon-bearingpowder”) as a feedstock, due to the problem of the strength being harderto raise compared with pellets made of only a metal oxide powder.

[0016] As methods of obtaining a high strength shaped article having nopowder, there are the method of producing spherical green pellets by apan type pelletizer, the method of forming briquets by shaping by a moldetc., and the method of extrusion of a type extruding a shaped articlefrom a perforated plate. Among these, green pellets obtained by a pantype pelletizer have the merit of inexpensive production of dense, highstrength green pellets. The pan type pelletization method is thereforefrequently used. With the conventional pelletization method, however,the only idea was to mix a powder metal oxide and a powder carbon sourceto form pellets. It was not always possible to produce high strengthgreen pellets for a rotary hearth reducing furnace or other reducingfurnace.

[0017] To meet with this demand, as prior art, for example, JapaneseUnexamined Patent Publication (Kokai) No. 11-193423 proposes a method ofimproving the strength of green pellets by mixing in an organic binderat the time of pelletization at the pan type pelletizer. However,sufficient consideration was not given to the technology relating to thedistribution of the particle size, ingredients, etc. of the feed powderand other feedstock conditions and the adjustment of moisture at thetime of pelletization and other operating conditions. This method didnot necessarily produce high strength green pellets. Further, greenpellets for use in rotary kilns or rotary hearth reducing furnaces andproduced from feed powder including coke fines etc. in a ratio of atleast 5% are particularly difficult to shape. Sometimes the strength canbe secured by addition of a binder, but in general the problem could notbe solved by just addition of a binder.

[0018] Further, Japanese Unexamined Patent Publication (Kokai) No.11-241125 discloses an apparatus for feeding dried green pellets to arotary furnace from a pelletizer through a pellet dryer. This is anapparatus which dries the green pellets of the feedstock in advance toprevent the green pellets from bursting due to moisture on the hightemperature hearth and is important technology. A method of productionof high strength green pellets which will not crumble when passedthrough the steps explained above and the configuration of a facilityfor the same have not yet however been elucidated.

[0019] The problems when pelletizing feed powder including acarbon-bearing powder are not limited to the strength of the greenpellets. When the feedstock conditions are poor, there is also theproblem of discontinuous ejection of green pellets from the pan typepelletizer. That is, when the distribution of the particle size of thefeed powder is poor or when adjustment of the moisture is improper, thegrowth of the green pellets in the pelletizer becomes unstable and thegreen pellets alternately are almost not ejected at all from thepelletizer and are ejected in large quantities. As a result, the feed ofthe green pellets to the reducing furnace connected to the downstreamsteps of the pelletizer becomes discontinuous and the problem of thereduction reaction becoming unstable arises. Further, the reduction instrength of the green pellets at the time when this phenomenon occurs isalso a major problem.

[0020] Further, even if the strength of the green pellets is high, ifthe green pellets are handled unsuitably, the green pellets will bebroken and powder will be generated during the screening or dryingoperation or during transport. Therefore, handling so as to preventcrumbling of the green pellets is also an important technology. In theconventional methods, however, this fact was not given sufficientattention. In the worst cases, 20 to 30% of the green pellets crumbledinto powder during transport or drying.

[0021] Further, the crumbling of the green pellets due to drying at thetime of feed into the hearth is also a problem. The green pelletsproduced by a pan type pelletizer are dense and high in strength in themoist state, but fall in strength when dried. Therefore, prevention ofcrumbling is important when dropping the dried green pellets on thehearth, but sufficient care has not been taken regarding this pointeither.

[0022] In this way, in the prior art, stable pelletization of feedpowder containing a carbon-bearing powder has been technicallydifficult. As a result, the operation of the reducing furnace has becomeunstable and there was the problem that efficient production of metalwas not possible.

[0023] Further, in a rotary hearth reducing furnace, it is necessary toproduce green pellets high in strength both in the wet state and the drystate. It is necessary to produce green pellets higher in strength thaneven the green pellets used for other purposes. Therefore, a newtechnology has been sought for stably producing high strength greenpellets using a powder containing a carbon-bearing powder as a feedstockand for realizing handling preventing crumbling of the same.

DISCLOSURE OF INVENTION

[0024] The present invention was made to solve the above problems andhas the following as its gist:

[0025] (1) Metal oxide-bearing green pellets for a reducing furnaceobtained by shaping a feed powder containing an oxide-bearing powder and5 to 30 wt % of a carbon-bearing powder, said feed powder containing 20to 80 wt % of particles of not more than 10 μm size.

[0026] (2) Metal oxide-bearing green pellets for a reducing furnaceobtained by shaping a feed powder containing an oxide-bearing powder and5 to 30 wt % of a carbon-bearing powder, said feed powder containing 20to 80 wt % of particles of not more than 10 μm size, and having aporosity of not more than 32%.

[0027] (3) A method of production of metal oxide-bearing green pelletsfor a reducing furnace characterized by pelletizing a feed powdercontaining a metal oxide-bearing powder and a carbon-bearing powder,wherein, when producing the green pellets, said feed powder includes 20to 80 wt % of powder particles of not more than 10 μm size.

[0028] (4) A method of production of metal oxide-bearing green pelletsfor a reducing furnace characterized by pelletizing a feed powdercontaining a metal oxide-bearing powder and 5 to 30 wt % of a drydistilled carbon-bearing powder, wherein, when producing the greenpellets, said feed powder includes 20 to 80 wt % of powder particles ofnot more than 10 μm size.

[0029] (5) A method of production of metal oxide-bearing green pelletsfor a reducing furnace characterized by pelletizing a feed powdercontaining a metal oxide-bearing powder and 10 to 35 wt % of fine coal,wherein, when producing the green pellets, said feed powder includes 20to 80 wt % of powder particles of not more than 10 μm size.

[0030] (6) A method of production of metal oxide-bearing green pelletsfor a reducing furnace characterized by pelletizing a feed powdercontaining a metal oxide-bearing powder and a total 10 to 60 wt % of twotimes the percent by weight of a dry distilled carbon-bearing powder andthe percent by weight of fine coal, wherein, when producing the greenpellets, said feed powder includes 20 to 80 wt % of powder particles ofnot more than 10 μm size.

[0031] (7) A method of production of metal oxide-bearing green pelletsfor a reducing furnace as set forth in any of paragraphs (3) to (6)characterized by mixing 0.5 to 4 wt % of bentonite or not more than 1 wt% of corn starch as a binder into said feed powder.

[0032] (8) A method of production of metal oxide-bearing green pelletsfor a reducing furnace as set forth in any of paragraphs (3) to (7)characterized by adjusting a moisture content in said feed powder beforecharging into a pelletizer and adjusting the moisture in said feedpowder held in the pelletizer to a range of 8 to 13 wt % forpelletization.

[0033] (9) A method of production of metal oxide-bearing green pelletsfor a reducing furnace as set forth in any of paragraphs (3) to (8)characterized in that said feed powder contains 15 to 75 wt % of dustcollected by a non-combustion type dust collector of converter gas andcollected as thickener precipitate.

[0034] (10) A method of production of metal oxide-bearing green pelletsfor a reducing furnace as set forth in any of paragraphs (3) to (8)characterized in that said feed powder contains 15 to 75 wt % of dustcontained in gas generated from an ironmaking electric furnace.

[0035] (11) A method of production of metal oxide-bearing green pelletsfor a reducing furnace as set forth in any of paragraphs (3) to (10)characterized in that the number of moles of atoms of carbon in saidfeed powder is 0.5 to 1.5 times the number of moles of atoms of oxygenof the metal oxide to be reduced by carbon in the range of 1200 to 1400°C.

[0036] (12) A method of production of metal oxide-bearing green pelletsfor a reducing furnace as set forth in any of paragraphs (3) to (11)characterized by drying said shaped green pellets to reduce the moisturecontent to not more than 2 wt %.

[0037] (13) A method of production of metal oxide-bearing green pelletsfor a reducing furnace as set forth in any of paragraphs (3) to (12)characterized by screening said shaped green pellets to removeundersized green pellets by a screen mesh of not less than 2 mm and toremove oversized green pellets by a screen mesh of not more than 30 mm.

[0038] (14) A method of reduction of metal oxide-bearing green pelletsfor a reducing furnace characterized by charging iron oxide-bearinggreen pellets for a reducing furnace as set forth in any of paragraphs(1) or (2) or iron oxide-bearing green pellets for a reducing furnaceproduced by a method as set forth in any of paragraphs (3) to (13) intoa zone of a rotary hearth reducing furnace having a furnace ambienttemperature of 900 to 1200° C. and reducing them by firing at atemperature of not less than 1200° C. for not less than 5 minutes.

[0039] (15) A method of reduction of metal oxide-bearing green pelletsfor a reducing furnace as set forth in paragraph (14) characterized bycharging onto the hearth of the rotary hearth reducing furnace andreducing by firing green pellets having a mean diameter of 8 to 20 mm ina mean number of layers of not more than 2.0.

[0040] (16) A method of reduction of metal oxide-bearing green pelletsfor a reducing furnace characterized by pelletizing a feed powdercomprised of a metal oxide-bearing powder and a carbon-bearing powder,including 20 to 80 wt % of powder particles having a size of not morethan 10 μm, to make green pellets, screening the green pellets by ascreen to remove undersized and oversized green pellets, reducing themoisture content by a pellet dryer, then charging the green pellets intoa rotary hearth reducing furnace and reducing by firing the greenpellets while continuously conveying green pellets between unitoperations.

[0041] (17) A reducing facility for metal oxide-bearing green pelletsfor a reducing furnace comprised of a pan type pelletizer, a screen, apellet dryer, and a rotary hearth reducing apparatus arranged in thatorder and provided with a continuous conveyor among them to link thesame.

[0042] (18) A reducing facility for metal oxide-bearing green pelletsfor a reducing furnace comprised of a plurality of powder storage tankshaving feeders controllable in feed rate, a kneader, a pan typepelletizer, a screen, a continuous pellet dryer, and a rotary hearthreducing furnace arranged in that order, provided with a moisture adderat one or both of the space between said powder storage tanks and saidkneader and the space between said kneader and said pan type pelletizer,and provided with a continuous conveyor between the apparatuses from thepowder storage tanks to the rotary hearth reducing furnace to link thesame.

[0043] (19) A reducing facility for metal oxide-bearing pellets for areducing furnace as set forth in any of paragraphs (17) or (18)characterized in that the total of the dropped heights of the pelletsfrom said pan type pelletizer to said pellet dryer is not more than 7 mand the total of the dropped heights of the pellets from said pelletdryer to said rotary hearth reducing furnace is not more than 4 m.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044]FIG. 1 is an overall flow chart of a rotary hearth reducingfacility as an example of working of the present invention and shows theflow from the step of preparation of the feedstock to the step ofreduction.

[0045]FIG. 2 is a view of a cross-section of a rotary hearth reducingfurnace.

BEST MODE FOR CARRYING OUT THE INVENTION

[0046] The present invention relates to green pellets made of powdercontaining metal oxide and carbon for a vertical type shaft furnace,rotary kiln, rotary hearth reducing furnace, or other reducing furnaceusing green pellets of a feed powder, a method of production of thesame, a method of reduction of green pellets in such a reducing furnace,a facility for the same, and other technology. Here, the explanationwill be given of the example of a rotary hearth method requiring thegreatest strength of the green pellets, but the invention is not limitedto a rotary hearth reducing furnace and can be applied to other reducingfurnaces as well.

[0047] As an example of the method of operation of the presentinvention, a general overview of a rotary hearth reduction process isshown in FIG. 1. This facility is mainly comprised of a plurality offeed storage bins 1, a kneader 5, a pan type pelletizer 7, a pelletscreen 9, a pellet dryer 11, and a rotary hearth reducing furnace 13.Note that in the present specification, the example of installation of aball mill type kneader and a hot air type pellet dryer using waste heatis shown.

[0048] Two or more types of feedstock, that is, the feed powderincluding the metal oxide and carbon, are used to control thepelletizability, chemical ingredients for the reduction, and otherrequired properties. Speaking from the viewpoint of the exampleperformed by this facility, three types of powder, that is, relativelycoarse iron oxide, fine iron oxide, and coarse coke fines were used.Iron ore fines called “pellet feed” consists of somewhat coarseparticles for a rotary hearth reducing furnace and contains largeamounts of iron oxide. Further, converter dust consists of fineparticles and contains large amounts of iron oxide, Coke fines consistsof coarse particles and contains large amounts of carbon. These threetypes of materials are used and placed in separate storage bins 1.

[0049] The ratios of mixture of the feedstocks are determined and theplurality of feedstocks are fed out onto the feed conveyor 2 from theplurality of feed storage bins 1. For this, the powder feeders 29 of thefeed storage bins 1 have to have function of constant feeding withvariable set value. The ratios of mixture are determined mainlyadjusting the particle size, chemical ingredients, and moisture content.To increase the strength of the pellets, a suitable distribution ofparticle size is required, so coarse powder and fine powder are mixed inpredetermined ratios. Further, to ensure a suitable reduction reaction,suitable ratios of the metal oxide and carbon are set. The powders arefed out onto the feed conveyor 2 in predetermined ratios mainly so as tosatisfy the above two matters.

[0050] The ratio of carbon content in the feed powder is determined bythe oxygen (active oxygen) chemically bonded with the metal oxide to bereduced. That is, an iron, nickel, or other oxide is reduced by carbonin a rotary hearth reducing furnace 13 at around 1200° C. to 1400° C.With reduction in a rotary hearth reducing furnace, the reductionreaction is mostly under conditions where the metal oxide and carbonform carbon monoxide. Therefore, the ratio of the carbon in the feedpowder and the metal oxides including iron oxides is preferably 0.5 to1.5 based on a center criteria of 1.0 of the ratio of the number ofmoles of atoms of carbon to the number of moles of atoms of activeoxygen in these metal oxides. Note that the ratio of mixture of thecarbon at this time is about 5 to 25 wt %.

[0051] As explained above, the carbon source used is coal, coke,charcoal, pitch, etc. In the case of coal, solid carbon and carbonincluded in volatile ingredients are included. The solid carboncontributes to the reduction reaction effectively, but the carbon in thevolatile ingredients ends up volatilizing before the reduction reactionstarts, so is not effectively utilized for the reduction. Therefore, anexcess of carbon of coal is required as compared to the amount of carbonrequired for reduction and the ratio of mixture into the feed powderbecomes greater.

[0052] Note that the moisture content is set so as to be lower than thesuitable moisture content in the kneader 5 or the pan type pelletizer 7.Therefore, the chemical ingredients of the feedstock, distribution ofparticle size, moisture content, etc. are measured in advance.

[0053] When the moisture content of the feed powder placed on the feedconveyor 2 is not more than the suitable moisture content at the kneader5, a moisture adder 3 is used to add moisture and a moisturized feedconveyor 4 is used to send the powder to the kneader 5. The ratios ofmixture of the feed powders is determined with first priority given tothe particle size and chemical ingredients, so the moisture of the feedpowder is not necessarily in a range suitable for the kneader 5.Further, the moisture in the feed powder fluctuates by the weather andother conditions. Therefore, adjustment of the moisture at this stage isextremely important.

[0054] The kneader 5 homogeneously mixes the feed powder. At that time,if pulverizing the feed powder lightly, the production of the pellets inthe pelletization step becomes stabler and the strength of the pelletsis improved, so a kneader having a pulverizing function such as a ballmill is desirable. For example if using a kneader comprised of acylinder, a cylinder rotating apparatus, and metal balls filled in thecylinder, it is possible to charge the feed powder into the cylinder androtate the cylinder to knead the feed powder and pulverize the feedpowder by the metal balls. There is a suitable moisture content of thepowder for the type and size of the kneader 5. In the case of a ballmill, the moisture content is desirably in the range of about 6 to 9%.

[0055] The feed powder finished being kneaded is sent by a kneaded feedconveyor 6 to the pan type pelletizer 7. There, the feed powder istumbled in a disk-shaped pan of a diameter of 2 to 6 m inclined about 45degrees so that powder coats the nuclei generated and green pellets ofseveral mm to about 30 mm size are produced. To produce high strengthgreen pellets by a stable productivity in the pan type pelletizer, it isnecessary that the distribution of particle size and moisture content ofthe feed powder be suitable. Note that generally the strength of thegreen pellets required for green pellets for a reducing furnace such asa rotary hearth reducing furnace is, in terms of crushing strength, atleast 2×10⁵ N/m². Further, such green pellets should have a 50 cm dropstrength, defined by the number of times before cracking when dropped 50cm, of at least 7 times, preferably 10 to 15 times, in the moist stateand at least 3 times, preferably 4 to 8 times, in the dry state. Themethod of producing green pellets meeting these conditions will bedescribed below.

[0056] To produce green pellets with a stable productivity and highstrength as explained above, it is important first of all that thedistribution of particle size of the feed powder be suitable. Forexample, even with the conventional pelletization technology, thepresence of at least 60 wt % of particles of not more than 74 μm sizewas a required condition.

[0057] If however including a carbon-bearing powder like the feed powderhandled in the present invention, with pelletization by the prior art, astable pelletization operation and production of high strength greenpellets were not possible. That is, carbon-bearing powder has a pooraffinity with water and weakens the bonds between the feed powders inthe green pellets. The carbon-bearing powder is poor in bonding withsurrounding particles, so even if the distribution of particle sizes issubstantially the same, green pellets containing carbon-bearing powderwill be relatively low in strength. It was confirmed that the higher theratio of the carbon-bearing powder, the lower the strength of the greenpellets.

[0058] In the conventional pelletization method, when mixing a drydistilled powder such as coke or charcoal as the carbon-bearing powderin a ratio of more than 5 wt %, high strength pellets could not beproduced. Further, in the case of coal, the limit value was 10 wt %.

[0059] The present inventors engaged in various experiments on the caseof inclusion of at least 5 wt % of a carbon-bearing powder or at least10 wt % of coal and as a result discovered that there are large spacesaround carbon-bearing powder in green pellets and as a result thecrushing strength is low. Therefore, the present inventors learned thatit is important to fill the spaces and that use of a material with asuitable distribution of particle sizes results in dense powder insidethe pellets, a higher strength of the pellets, a uniform size of greenpellets in the pan type pelletizer, and a constant ejection rate.

[0060] While investigating the suitable distribution of particle sizes,the inventors mixed powder of not more than 10 μm size into the feedpowder and pelletized the feed powder, whereby the small powder granulessurrounded the relatively large size carbon-bearing powder, the greenpellets became denser, and the strength of the green pellets wasimproved.

[0061] That is, it was learned that when fine particles and coarseparticles are mixed in certain ratios, high strength green pellets canbe produced.

[0062] Further, repeated experiments showed that if particles of notmore than 10 μm size are present in an amount of at least 20 wt % in thefeed powder as fine particles, the powder filling rate of the greenpellets rises and it is possible to realize the strength sought forgreen pellets for reducing furnaces. Further, the size of the greenpellets was made uniform in the pan type pelletizer 7, the discharge ofgreen pellets from the pan type pelletizer 7 became uniform in speed,and the pelletization operation became stable. On the other hand, if theamount of the particles of not more than 10 μm size in the feed powderbecomes more than 80 wt %, nuclei are not formed well inside the pantype pelletizer 7, the growth of the green pellets becomes slower, andonly green pellets of a low density, insufficient strength, and unstablesize can be produced. This is because adding too much fine particlesconversely obstructs the densification. Therefore, feed powderconsisting of the same feedstock, but having at least 20 wt % ofparticles of not less than 10 μm size was pelletized, whereby it waspossible to produce high strength green pellets with a stable grainsize.

[0063] That is, to produce dense, high strength green pellets, it isimportant that coarse particles and fine particles be mixed together insuitable ratios. That the ratio of particles of not more than 10 μm sizecomprise 20 to 80 wt % of the feed powder is an important condition.

[0064] However, if the ratio of the carbon-bearing powder in the feedpowder is extremely large, the effect of mixing in fine particles of thefeed powder to raise the strength of the green pellets also falls. Inthe case of dry distilled carbon-bearing powder treated, if the ratio ofmixture exceeded 30 wt %, even if the ratio of particles of not morethan 10 μm size was suitable, the strength of the green pellets did notexceed the above required value. Further, in the case of coal, if theratio of mixture exceeded 35 wt %, the strength of the pellets did notexceed the above required value. Therefore, the ratio of thecarbon-bearing powder in the feed powder of the present invention is 5to 30 wt % in the case of a dry distilled carbon-bearing powder and in arange of 10 to 35 wt % in the case of coal. Further, when using a drydistilled carbon-bearing powder and coal mixed together, the total oftwo times the ratio of the dry distilled carbon-bearing powder and theratio of the coal is in the range of 10 to 60 wt %.

[0065] There are various methods for adjusting the ratio of the feedpowder of not more than 10 μm size. The easiest is to adjust the ratioof mixture of the feed powder having a high ratio of particles of notmore than 10 μm size. As such powder, it is best to use dust collectedas thickener precipitate from a non-combustion type dust collector ofconverter gas, that is, converter dust. Converter dust contains 80 to 90wt % of particles of not more than 10 μm size and is desirable as asource of fine particles. Further, it has a high content of at least 70wt % of iron, so there is also the effect that after reduction,production of good quality reduced pellets having a high iron content ispossible. The ratio of mixture of the converter dust is preferably 15 to75 wt %. Further, the dust contained in gas generated from an ironmakingelectric furnace, that is, electric furnace dust, has similar effects.The ratio of mixture is preferably 15 to 75 wt %. However, electricfurnace dust has a smaller content of iron, so is not efficient forproduction of reduced pellets having a high iron content.

[0066] In the pan type pelletizer, to produce high strength greenpellets with a stable productivity, it is necessary that the moisturecontent be suitable in addition to the distribution of particle sizes ofthe feed powder. Therefore, it is necessary to finely control themoisture content of the feed powder in the pan type pelletizer 7. If themoisture content is too low, the growth of the green pellets will beslow and dense, high strength green pellets cannot be produced. Further,if the moisture content is too great, the small sized green pelletsstarting to grow will stick to each other resulting in green pellets ofabnormal shapes and extremely low strength. In this state, further,pellets will no longer stably be produced from the pelletizer and greenpellets will be intermittently discharged. As a result, the amountprocessed per unit time in the downstream steps of the pellet dryer 11and the rotary furnace 13 will fluctuate in a short time and the overallprocess operation will become unstable.

[0067] Therefore, the present inventors sought the moisture contentvalue suitable for the pan type pelletization method and as a resultdiscovered that while differing depending on the type or size of thefeed powder, there was a suitable value between 8 to 13 wt %. However,unless the fluctuation in the moisture content is made not more than 2%with the same type and size of the feed powder, the above-mentionedproblems arise and pelletization becomes unstable. Therefore, it isimportant to adjust the feed powder before charging into the pan typepelletizer. For example, it is important to adjust it to a moisturecontent suitable for pelletization in the kneading step. When thesuitable moisture content in the kneading step is lower than themoisture content value in the pelletization step, the moisture contentis adjusted to a suitable range by a moisture adder, not shown in FIG.1, between the kneading step and the pelletization step.

[0068] Further, depending on the composition of the feedstock, even ifthe distribution of the particle sizes is suitable, sometimes thestrength of the green pellets is low and sometimes it is desired toraise the strength of the green pellets. In such cases, it is effectiveto mix in a binder. The present inventors discovered that bentonite andcorn starch are binders which do not give off gas or moisture creatingobstacles when reducing green pellets in a high temperature furnace. Thesuitable ratio of mixing of these binders is 0.5 to 4 wt % with respectto the feed powder for bentonite and not more than 1 wt % for cornstarch. If the binder is mixed in with not less than this ratio, thesame phenomenon occurs as when there is too much moisture, i.e., thesmall sized pellets stick together in the growth process and problemsarise in the stability of the pelletization operation and the pelletstrength.

[0069] If suitably producing green pellets by the above method, it ispossible to produce dense green pellets having a porosity of not morethan 32%. As a result, it is possible to produce pellets having acrushing strength of 2×10⁵ N/m², a 50 cm drop strength in the moiststate of 7 to 15 times, and a 50 cm drop strength in the dry state of 3to 8 times. This satisfies the usage conditions in a reducing furnace.

[0070] The green pellets produced by the above method are screened toremove the powder and the large sized green pellets, then the pelletsare dried by a dryer 11 and reduced by firing in a rotary furnace 13.

[0071] Note that the green pellets produced by the pan type pelletizer 7contain small sized green pellets and powder as well. The small sizedgreen pellets and powder cause the problem of obstruction of the flow ofgas between the green pellets in the pellet dryer 11, the problem ofbuildup on the hearth of the rotary furnace, etc. Therefore, the greenpellets are sent to a pellet screen 9 by a green pellet conveyor 8 wherethe small sized green pellets and powder are removed. If the screen meshis too fine, the screen will easily clog, so the screen mesh is made atleast 2 mm.

[0072] This pellet screen 9 is an effective means for removing oversizedgreen pellets as well. The larger the green pellets, the lower thestrength. Further, overly large green pellets take time for heat to beconducted to their insides and therefore have the problem of a longerreduction time in a rotary furnace. In particular, from the viewpoint ofsecuring the strength of the green pellets, it is effective to removegreen pellets of not less than 30 mm size.

[0073] Placing the pellet screen 9 before the pellet dryer 11 isimportant in the present invention. The reason is that generally whenscreening something by a screen, the operation is performed by amechanical means such as vibration. Green pellets in the moist statehave a relatively high strength, so can withstand such mechanicalvibration. That is, if screening green pellets in the moist state by ascreen, there are few green pellets which will crumble. However, driedgreen pellets are low in strength and often crumble. Therefore, asdescribed in FIG. 1, it is effective to screen green pellets in themoist state.

[0074] The green pellets after screening are sent to the pellet dryer 11by a post-screen conveyor 10 where they are then dried. As the type ofapparatus, a hot air type dryer is preferable. Further, in the sense ofenergy savings, it is economical to create hot air at a heat exchanger18 for recovering the sensible heat of the exhaust gas of the rotaryfurnace and use this for the drying. To prevent explosion of the greenpellets along with evaporation of moisture, the drying is performed at alow temperature of not more than 250° C.

[0075] Note that after producing the green pellets by the pan typepelletizer 7, it is important to continuously process and transport andnot to store the green pellets up until being fed on to the hearth 26 ofthe rotary furnace 13. Further, it is necessary to continuously processthe green pellets in the pellet screen 9 and pellet dryer 11. In thecase of batch processing, pellet storage tanks, dischargers, etc. arerequired before and after intermittent processing. If storing greenpellets in a yard or bins, the weight of the green pellets on the topwill crush the green pellets on the bottom or the shovel car or feederwill impart mechanical shock at the time of discharging and the greenpellets will easily crumble.

[0076] Further, for these reasons, transport from the pan typepelletizer 7 to the rotary furnace 13 is preferably by a continuousconveyor such as a green pellet conveyor 8, post-screen conveyor 10, drypellet conveyor 12, etc. without storing the green pellets. As thecontinuous conveyor, a belt conveyor or a pipe conveyor is effective.This is because these do not apply an unnecessary force to the greenpellets during conveyance. Further, if a short distance, conveyance by avibration type conveyor is also possible. It is desirable to use theseconveyors at suitable locations making use of these particularproperties.

[0077] That is, as shown in FIG. 1, by arranging a plurality of powderstorage tanks 1 having feeders 29 capable of being controlled in feedrate, a kneader 5, a pan type pelletizer 7, a pellet screen 9, a pelletdryer 11, and a rotary hearth reducing furnace 13 in that order andfurther providing a moisture adder in one or both of the space betweenthe powder storage tanks 1 and the kneader 5 and the space between thekneader 5 and the pan type pelletizer 7, providing for example a feedconveyor 2, green pellet conveyor 8, post-screen conveyor 10, dry pelletconveyor 12, or other continuous conveyors between these apparatusesfrom the powder storage tanks 1 to the rotary hearth reducing furnace13, and, when providing a moisture adder 3 between the powder storagetanks 1 and the kneader 5, providing a moisturized feed conveyor 4between the kneader 5 and the moisture adder 3, while, when providing anot shown moisture adder between the kneader 5 and the pan typepelletizer 7, providing a continuous conveyor of any not shownmoisturized kneaded feed conveyor between the moisture adder and pantype pelletizer, the moisture content at the time of production of thegreen pellets is efficiently adjusted and the green pellets efficientlytransported.

[0078] Further, even when operating suitably by the above processing andtransport methods, with the strength of green pellets produced by thepan type pelletizer 7, if the total dropped distances during transportis long, the pellets will be destroyed by the drop impact. The presentinventors found that the strength of the green pellets was a maximum of15 times at 50 cm drops before drying and a maximum of 8 times at 50 cmdrops in the dry state and that, considering from the strength of thegreen pellets, the total dropped distances from the pan type pelletizer7 to the pellet dryer 11 of the green pellets in the moist state had tobe not more than 7 m and the total dropped distances from the pelletdryer 11 to the hearth 25 of the rotary furnace 13 of the green pelletsin the dry state had to be not more than 4 m.

[0079] Next, the dried green pellets are sent by the dry pellet conveyor12 to the rotary furnace 13 where they are reduced by firing.

[0080] In ordinary operation of a rotary furnace, the temperature of thehearth is 1000 to 1150° C. and the ambient temperature of the heatingzone is 900 to 1200° C.

[0081] The dried green pellets are fed to a portion of the rotaryfurnace where the ambient furnace temperature is 900 to 1200° C. Byfeeding dense green pellets having a porosity of not more than 32% tosuch a high temperature atmosphere, there is a danger of bursting due tothe evaporation of moisture inside the green pellets. Therefore, toprevent the dense green pellets having a porosity of not more than 32%from bursting due to moisture under such temperature conditions, it isimportant that the moisture content of the green pellets be low. Thepresent inventors investigated the moisture content at which greenpellets produced by the method of the present invention would not burstunder these conditions and as a result found there was no problem if themoisture content is not more than 2 wt %. Therefore, it is effective toreduce the moisture content of the green pellets after drying to notmore than 2%.

[0082] As shown in FIG. 2, the rotary furnace 13 is comprised instructure of a rotary type hearth 25 moving above a wheel 27 below aceiling 22 and furnace walls 23. The green pellets 28 are placedstationarily on the hearth 25 and circle the inside of the furnace. Inthe furnace, combustion gas is burned from a burner 24. The maximumtemperature of the gas is made a suitable temperature between 1200° C.and 1400° C. The fed green pellets 28 initially enter the furnaceportion having a high oxidation degree of the gas and 900 to 1200° C. intemperature (heating zone) where they are heated. Next, the greenpellets are reduced at the portion with a low gas oxidation degree andhigh temperature (reducing zone).

[0083] In the reducing zone, the temperature at which the iron, nickel,or other metal oxide reacts strongly with carbon and is reduced is atleast 1200° C., so the fed green pellets are fired by heating to atleast 1200° C., whereupon the metal oxide and carbon react and thereduced metal and carbon monoxide are produced. Further, zinc, lead, andother metals having a high vapor pressure at about 1200° C. are removedby evaporation from the green pellets. The reduction time is a minimumof 5 minutes. After reduction for 5 to 20 minutes, the iron or nickel orother relatively easily reduced metal of the green pellets is reduced.The heat transfer of the rotary furnace 13 is the ratiotion of the hightemperature gas above the green pellets and the heat conduction from thehearth 26. Therefore, with up to two stacked layers of green pellets,heat is conducted directly from either the top or the bottom, but withnot less than two stacked layers of green pellets, the green pellets inthe middle do not easily directly receive heat and the reductionreaction is slower. Therefore, the mean number of layers of pellets isdesirably not more than 2.0. If considering the mode and speed of heattransfer in the rotary furnace 13 in this way, the mean size of thegreen pellets is preferably 8 to 20 mm. If the mean size of the greenpellets is not more than 8 mm, the productivity per hearth area falls,while if the mean size of the green pellets is not less than 20 mm, theheat is transferred slower inside the green pellets and therefore thereduction reaction of the center parts will not end with a reaction timeof about 5 minutes.

[0084] The reduced pellets finished being reduced are discharged fromthe rotary furnace 13 by an discharger. The reduced pellets dischargedfrom the rotary furnace are sent to the melting step in the hightemperature state or cooled at the reduced pellet cooling apparatus 14and sent via the reduced pellet conveyor 15 to the reduced pelletstorage tank 16. In FIG. 1, the example of provision of a facility forcooling the reduced pellets is shown. Next, the green pellets are sentto a step using them in for example a blast furnace, electric furnace,or converter.

[0085] Note that the combustion exhaust gas from the rotary furnace issent from an exhaust duct 17 to the heat exchanger 18 where it is usedto heat air. The air is used as a heat source for drying the greenpellets. Next, the combustion exhaust gas is cleaned of dust by the dustcollector 19 and discharged from the chimney 20 to the atmosphere.

[0086] Green pellets produced by the method of the present invention canbe used for reduction not only in a rotary hearth reducing furnace, butalso reduction by a rotary kiln or reduction in a low height verticalshaft furnace. In a rotary kiln, there is the effect of prevention offormation of a dam ring inside the kiln due to production of highstrength green pellets. Further, in a vertical shaft furnace, there isthe effect of prevention of obstruction to circulation of gas in thefurnace due to the generation of dust. That is, when using the reducedpellets in a blast furnace, it is necessary that the feedstock be grainshapes. That is, in a blast furnace, the flow rate of gas in the furnaceis fast, so there is a problem of spraying of dust etc. and feedstock ofa grain size of 3 to 5 mm or more is used. The reduced pellets accordingto the present invention have a high ratio of pellet shapes, so aresuitable for use in a blast furnace etc.

EXAMPLES Example 1

[0087] The results of operation using the facility of a rotary hearthreducing furnace shown in FIG. 1 are shown. The facility produces 15tons of reduced iron pellets for blast furnace use every hour. Therewere three materials: pellet feed powder ore, converter dust, and cokefines.

[0088] The pellet feed ore contained 89 wt % of ferric oxide (Fe₂O₃),had a mean particle size of 68 μm, and had a ratio of particles of notmore than 10 μm size of 13 wt %. Further, the converter dust contained34 wt % of iron oxide (FeO) and 43 wt % of metal iron, had a meanparticle size of 6 μm, and had a ratio of particles of not more than 10μm size of 81 wt %. The coke dust fines contained 83 wt % of carbon, hada mean particle size of 89 μm, and had a ratio of particles of not morethan 10 μm size of 8 wt %.

[0089] In Example 1, 40 wt % of pellet feed powder ore, 37 wt % ofconverter dust, and 23 wt % of coke dust fines were mixed and fed out onto a feed transport conveyor 2. The ratio of particles of not more than10 μm size was 36 wt %. Further, the molar ratio of atoms of carbon andoxygen bonded with the iron oxide was 0.86. The ratio of powder of notmore than 10 μm size in the feed powder was in the range of 20 to 80 wt%, so the method of mixing was according to the present invention.

[0090] The moisture content of this mixed feed powder was 7 to 8 wt %,so water was sprinkled to add moisture in advance to obtain a moixturecontent of about 9 wt %, then about 1 wt % of moisture was added bysprinkling by the pan type pelletizer 7 to stabilize pelettization. Bythis method, the moisture content was made a suitable moisture contentfor the pan type pelletizer 7, that is, 9.5 to 11 wt %. Note that inExample 1, as the binder, bentonite was added in an amount of 1.4 wt %of the weight of the feed powder. The shaped green pellets had a meandiameter of 13.4 mm and a mean crushing strength of 2.9×10⁵ N/m² andtherefore were green pellets with a high strength. Further, the 50 cmdrop strength of the green pellets was 9 times in the moist state and 4times in the dry state.

[0091] The green pellets were dried and fed to a rotary furnace 9.During this time, the green pellets crumbling before reduction accountedfor 7.5 wt % of the total. The grain ratio over 4 mm of the reducedpellets after 12 minutes at a maximum temperature of 1320° C. was 92 wt% and the metallization rate was a good 92 wt %.

[0092] Next, as Comparative Example 1, the results when operating basedon a conventional method are shown. The facility used was that of FIG.1, but the method of operation was the same as in the past. As thefeedstock, a feed powder comprised of 74 wt % of the above-mentionedpellet feed powder ore and 26 wt % of coke dust fines mixed together wasused. The ratio of particles having a size of not more than 10 μm atthis time was 12 wt %. Further, the molar ratio of atoms of carbon andoxygen bonded with the iron oxide was 1.0.

[0093] This feed powder was adjusted in moisture content, then shapedinto green pellets by the pan type pelletizer 7. The method of operationwas the same as in Example 1. As a result, green pellets of a meandiameter of 12.8 mm were obtained, but the mean crushing strength was1.3×10⁵ N/m², so the strength was low. Further, the 50 cm drop strengthof the green pellets was 5 times in the moist state and 1 time in thedry state.

[0094] The green pellets were screened, dried, and reduced in the sameway as in Example 1. As a result, the green pellets crumbling beforereduction accounted for a large 19.8 wt % of the total. Further, thegrain ratio over 4 mm of the reduced pellets was a small 78 wt % and themetallization rate was a low 78 wt %. Since the ratio of green pelletscrumbling during operation or transport was large or the ratio of greenpellets becoming powder on the hearth 25 was large in this way, thepowder reoxidized in the furnace or after discharging and themetallization rate fell by a large extent.

[0095] In this way, in Example 1 of operation using the presentinvention, there were few green pellets crumbling in the middle of theprocessing and the operation was performed with a high ratio andmetallization rate of the reduced pellets as a product. On the otherhand, in Comparative Example 1, these results were poor.

Example 2

[0096] The results of operation using the facility of a rotary hearthreducing furnace shown in FIG. 1 are shown. The facility produces 15tons of reduced iron pellets for blast furnace use every hour. Therewere three material storage bins containing pellet feed powder ore,converter dust, and coke dust fines of a coke dry quencher.

[0097] The pellet feed ore contained 89 wt % of ferric oxide (Fe₂O₃),had a mean particle size of 68 μm, and had a ratio of particles of notmore than 10 μm size of 13 wt %. Further, the converter dust contained34 wt % of iron oxide (FeO) and 43 wt % of metal iron, had a meanparticle size of 6 μm, and had a ratio of particles of not more than 10μm size of 81 wt %. The coke dust fines contained 83 wt % of carbon, hada mean particle size of 89 μm, and had a ratio of particles of not morethan 10 μm size of 8 wt %.

[0098] In Example 2, 40 wt % of pellet feed powder ore, 37 wt % ofconverter dust, and 23 wt % of coke dust fines were mixed and fed out onto a material transport conveyor 2. The mean particle size of this mixedfeed powder was 50 μm, while the ratio of particles of not more than 10μm size was 36 wt %. Further, the molar ratio of atoms of carbon andoxygen bonded with the iron oxide was 0.86. The ratio of powder of notmore than 10 μm size in the feed powder was in the range of 20 to 80 wt%, so the method of mixing was according to the present invention.

[0099] The moisture content of this feed powder was 5.7 wt %. On theother hand, the moisture content value enabling suitable kneading by theball mill type kneader 5 of FIG. 1 is 7 to 9 wt %, so water wassprinkled to obtain a moisture content of 8.2 wt % by a moisture adder3. Note that with a ball mill, if the moisture content is too low, themixing is poor, while if the moisture content is too high, powder sticksto the inside wall of the cylinder, so it is important to control themoisture to this range. Here, the feed powder was sufficiently mixed andsent on to the next pelletization step where the feed powder waspelletized by the pan type pelletizer 7. The moisture content valuesuitable for pelletization in Example 2 was 9.5 to 11 wt %. Further,with the pan type pelletizer 7, pelletization was stabilized bysprinking 1 to 1.5 wt % of water there. In Example 2, the moisturecontent of the feed powder before entering the pan type pelletizer 7 wasin a suitable range, so no moisture was added before the pan typepelletizer 7. When the moisture content of the feed powder is low,however, moisture is added. Note that in Example 2, as the binder,bentonite was added in an amount of 1.4 wt % of the weight of the feedpowder.

[0100] The shaped green pellets had a mean diameter of 13.4 mm and amean crushing strength of 2.9×10⁵ N/m² and therefore were green pelletswith a high strength. Further, the 50 cm drop strength of the greenpellets was 9 times in the moist state and 4 times in the dry state.

[0101] The green pellets were screen by a pellet screen 9 to removepellets of not more than 5 mm or not less than 25 mm size, then weredried by a pellet dryer 11 and fed to a rotary furnace 13, During thistime, the green pellets crumbling before reduction accounted for 7.5 wt% of the total. The grain ratio over 4 mm of the reduced pelletsdischarged from the furnace after reduction for 12 minutes at a maximumtemperature of 1320° C. was 92 wt % and the metallization rate was agood 92 wt %.

[0102] Next, as Comparative Example 2, the results when operating basedon a conventional method are shown. The facility used was that of FIG.1, but the method of operation was the same as in the past. As thefeedstock, a feed powder comprised of 74 wt % of the above-mentionedpellet feed powder ore and 26 wt % of coke dust fines mixed together wasused. The ratio of particles having a size of not more than 10 μm atthis time was 12 wt %. Further, the molar ratio of atoms of carbon andoxygen bonded with the iron oxide was 0.86.

[0103] This feed powder was adjusted in moisture content, then shapedinto green pellets by the pan type pelletizer 7. The method of operationwas the same as in Example 2. As a result, green pellets of a meandiameter of 12.8 mm were obtained, but the mean crushing strength was1.3×10⁵ N/m², so the strength was low. Further, the 50 cm drop strengthof the green pellets was 5 times in the moist state and 1 time in thedry state.

[0104] The green pellets were screened, dried, and reduced in the sameway as in Example 2. As a result, the green pellets crumbling beforereduction accounted for a large 19.8 wt % of the total. Further, thegrain ratio over 4 mm of the reduced pellets was a small 78 wt % and themetallization rate was a low 78 wt %. Since the ratio of green pelletscrumbling during operation or transport was large or the ratio of greenpellets becoming powder on the hearth 25 was large in this way, thepowder reoxidized in the furnace or after discharging and the metalconversion rate fell by a large extent.

[0105] In this way, in Example 2 of operation using the presentinvention, there were few green pellets crumbling in the middle of theprocessing and the operation was performed with a high grain ratio andmetallization rate of the reduced pellets as a product. On the otherhand, in Comparative Example 2, these results were poor.

INDUSTRIAL APPLICABILITY

[0106] By using the green pellets of the present invention, method ofproduction of the same, method of reduction of the same, and reductionfacility, it is possible to produce high strength green pellets able towithstand use in a reducing furnace. It is possible to reduce the greenpellets by firing in a reducing furnace such as a rotary kiln or rotaryhearth reducing furnace without crumbling the pellets and possible toobtain reduced pellets high in grain ratio and high in reduction rate.By using the pellets reduced by this reducing furnace, a reduced metalcan be efficiently produced.

1. Metal oxide-bearing green pellets for a reducing furnace obtained by shaping a feed powder containing an oxide-bearing powder and 5 to 30 wt % of a carbon-bearing powder, said feed powder containing 20 to 80 wt % of particles of not more than 10 μm size.
 2. Metal oxide-bearing green pellets for a reducing furnace obtained by shaping a feed powder containing an oxide-bearing powder and 5 to 30 wt % of a carbon-bearing powder, said feed powder containing 20 to 80 wt % of particles of not more than 10 μm size, and having a porosity of not more than 32%.
 3. A method of production of metal oxide-bearing green pellets for a reducing furnace characterized by pelletizing a feed powder containing a metal oxide-bearing powder and a carbon-bearing powder, wherein, when producing the green pellets, said feed powder includes 20 to 80 wt % of powder particles of not more than 10 μm size.
 4. A method of production of metal oxide-bearing green pellets for a reducing furnace characterized by pelletizing a feed powder containing a metal oxide-bearing powder and 5 to 30 wt % of a dry distilled carbon-bearing powder, wherein, when producing the green pellets, said feed powder includes 20 to 80 wt % of powder particles of not more than 10 μm size.
 5. A method of production of metal oxide-bearing green pellets for a reducing furnace characterized by pelletizing a feed powder containing a metal oxide-bearing powder and 10 to 35 wt % of fine coal, wherein, when producing the green pellets, said feed powder includes 20 to 80 wt % of powder particles of not more than 10 μm size.
 6. A method of production of metal oxide-bearing green pellets for a reducing furnace characterized by pelletizing a feed powder containing a metal oxide-bearing powder and a total 10 to 60 wt % of two times the percent by weight of a dry distilled carbon-bearing powder and the percent by weight of fine coal, wherein, when producing the green pellets, said feed powder includes 20 to 80 wt % of powder particles of not more than 10 μm size.
 7. A method of production of metal oxide-bearing green pellets for a reducing furnace as set forth in any of claims 3 to 6 characterized by mixing 0.5 to 4 wt % of bentonite or not more than 1 wt % of corn starch as a binder into said feed powder.
 8. A method of production of metal oxide-bearing green pellets for a reducing furnace as set forth in any of claims 3 to 7 characterized by adjusting a moisture content in said feed powder before charging into a pelletizer and adjusting the moisture in said feed powder held in the pelletizer to a range of 8 to 13 wt % for pelletization.
 9. A method of production of metal oxide-bearing green pellets for a reducing furnace as set forth in any of claims 3 to 8 characterized in that said feed powder contains 15 to 75 wt % of dust collected by a non-combustion type dust collector of converter gas and collected as thickener precipitate.
 10. A method of production of metal oxide-bearing green pellets for a reducing furnace as set forth in any of claims 3 to 8 characterized in that said feed powder contains 15 to 75 wt % of dust contained in gas generated from an ironmaking electric furnace.
 11. A method of production of metal oxide-bearing green pellets for a reducing furnace as set forth in any of claims 3 to 10 characterized in that the number of moles of atoms of carbon in said feed powder is 0.5 to 1.5 times the number of moles of atoms of oxygen of the metal oxide to be reduced by carbon in the range of 1200 to 1400° C.
 12. A method of production of metal oxide-bearing green pellets for a reducing furnace as set forth in any of claims 3 to 11 characterized by drying said shaped green pellets to reduce the moisture content to not more than 2 wt %.
 13. A method of production of metal oxide-bearing green pellets for a reducing furnace as set forth in any of claims 3 to 12 characterized by screening said shaped green pellets to remove undersized green pellets by a screen mesh of not less than 2 mm and to remove oversized green pellets by a screen mesh of not more than 30 mm.
 14. A method of reduction of metal oxide-bearing green pellets for a reducing furnace characterized by charging iron oxide-bearing green pellets for a reducing furnace as set forth in any of claims 1 or 2 or iron oxide-bearing green pellets for a reducing furnace produced by a method as set forth in any of claims 3 to 13 into a zone of a rotary hearth reducing furnace having a furnace ambient temperature of 900 to 1200° C. and reducing them by firing at a temperature of not less than 1200° C. for not less than 5 minutes.
 15. A method of reduction of metal oxide-bearing green pellets for a reducing furnace as set forth in claim 14 characterized by charging onto the hearth of the rotary hearth reducing furnace and reducing by firing green pellets having a mean diameter of 8 to 20 mm in a mean number of layers of not more than 2.0.
 16. A method of reduction of metal oxide-bearing green pellets for a reducing furnace characterized by pelletizing a feed powder comprised of a metal oxide-bearing powder and a carbon-bearing powder, including 20 to 80 wt % of powder particles having a size of not more than 10 μm, to make green pellets, screening the green pellets by a screen to remove undersized and oversized green pellets, reducing the moisture content by a pellet dryer, then charging the green pellets into a rotary hearth reducing furnace and reducing by firing the green pellets while continuously conveying green pellets between unit operations.
 17. A reducing facility for metal oxide-bearing green pellets for a reducing furnace comprised of a pan type pelletizer, a screen, a pellet dryer, and a rotary hearth reducing apparatus arranged in that order and provided with a continuous conveyor among them to link the same.
 18. A reducing facility for metal oxide-bearing green pellets for a reducing furnace comprised of a plurality of powder storage tanks having feeders controllable in feed rate, a kneader, a pan type pelletizer, a screen, a continuous pellet dryer, and a rotary hearth reducing furnace arranged in that order, provided with a moisture adder at one or both of the space between said powder storage tanks and said kneader and the space between said kneader and said pan type pelletizer, and provided with a continuous conveyor between the apparatuses from the powder storage tanks to the rotary hearth reducing furnace to link the same.
 19. A reducing facility for metal oxide-bearing green pellets for a reducing furnace as set forth in any of claims 17 or 18, characterized in that the total of the dropped heights of the pellets from said pan type pelletizer to said pellet dryer is not more than 7 m and the total of the dropped heights of the pellets from said pellet dryer to said rotary hearth reducing furnace is not more than 4 m. 